Geodynamics of congested subduction zones - implications for evolution of the Tasmanides Pete Betts - School of Earth, Atmosphere, and environment, Monash University Louis Moresi – Department of Earth Sciences, University of Melbourne Ross Cayley – Geological Survey of Victoria Meghan Miller – University of Southern California Robin Armit David Willis New Perspectives 11 th September 2014
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Geodynamics of congested subduction zones -implications for evolution of the Tasmanides
Pete Betts - School of Earth, Atmosphere, and environment, Monash University
Louis Moresi – Department of Earth Sciences, University of Melbourne
Ross Cayley – Geological Survey of Victoria
Meghan Miller – University of Southern California
Robin Armit
David Willis
New Perspectives 11th September 2014
Congested subduction zones
• Mason et al, 2008; Betts et al, 2012; 2014; Moresi et al, 2014
showed how trench motion and slab
configuration are influenced by buoyant
material colliding with a subduction zone.
raised question: how does subduction
continue after accretion of one terrane ?
How does a microcontinent / plateau
switch to the over-riding plate ?
This is important at many different scales
Can we understand this process better and can
we apply this to understand places where
accretion is complete?
What does this mean for the evolution of the
Tasmanides?
Continental accretion & subduction
zone dynamics• Accretion is a vital component in
understanding how continents grow ...
• A number of poorly understood aspects of
accretion attacked using Underworld and
tested against the Tasmanides
How does a microcontinent / plateau switch to
the over-riding plate?
How does subduction step back & recover
(accretion v. collision)?
What is the signature in the over-riding plate
(i.e. in the geological record)?
2D v 3D interpretations
Accretionary
Orogens
Mechanism for Phanerozoic
continental growth and transfer
of crust.
Altiads – Central Asian Orogen
(closing an internal ocean)
Tasmanides – Facing an
external Ocean (paleo-Pacific
ocean).
By comparison the Tasmanides
is relatively simple from a
geodynamics and rock record
perspective.
After Collins et al. 2011
Nature Geoscience
1. Recognition of the Selwyn Block(Cayley and Taylor, 2002).
5. “Cayley model” in all its intricacies (Cayley in prep).
The Tasmanides
Recent Game changes – in my opinion
The Tasmanides
Turbidites
Granite rocks
Mafic-Ultramafic belts
Cambrian Turbidites
Neoproterozoic-Cambrian Rift and Cambrian back arc
Cambrian Rift and passive margin
New England
Orogen
Lachlan Orogen
North Queensland
(Mossman)
Thomson
Orogen
• Adelaide Fold Belt (Neoproterozoic-Ordovician).
• Lachlan Orogen (Neoproterozoic-Carboniferous).
• Thomson Orogen (Neoproterozoic – Triassic)
• New England Orogen (Cambrian-Triassic).
• North Queensland (Neoproterozoic-Triassic).
The Tasmanides – is the Murray River the most unusual
tectonic boundary on the planet?
After Glen et al. (2013)
After Gray and others (1990’s 2000’s)
Victoria
NSW
After Betts et al. (2012)
North QLD
The Tasmanides – It’s not really 3D
SW Pacific Margin
After Aitchison and Buckman (2012)
Oroclines are the flavour of the month!
Highlights the 3D problem
• Characterised by several large oroclinal features
• Silurian to Carboniferous
• Combination of roll-back and accretion.
LachlanOrocline
Bob Musgrave, 2009
NSW geological survey
High resolution geophysical data
VanDieland Micro-continent
VanDieland accretion
Selwyn Block
Modified after Cayley 2011 GR
VanDieland accretion
VanDieland accretion
VanDieland accretion – the Cayley (and Musgrave) model
VanDieland an embedded terrane
• The Australian accretion of the VanDieland micro-continent resulted in the terrane being deeply embedded in the over-riding plate and left largely undisturbed since then.
• Evidence of rotations in present day structural grain (from potential fields, paleomagnetism and other geological indicators).
Moresi, L., Betts, P. G., Miller, M. S., & Cayley, R. A.
(2014). Dynamics of continental accretion. Nature.
doi:10.1038/nature13033
• In 3D a small buoyant block is easily accreted or eaten by the subduction zone. How about a large one ?
How does the slab recover from accretion with break-off / windowing ?
What should we look for in the superficial geological record ?
What happens for a large terrane/microcontinent ?
2D thinking is misleading
Size does matter!
Model set-up – USING UNDERWORLD• Layer 1 density accounts for ~7km oceanic crust (but not phases changes during
subduction)
• Layer 1 yield strength is (very) low to account for (unresolved) near-surface
faulting, entrainment of sediments into the plate boundary & crust
• Viscosity is truncated after averaging (to 105 x asthenosphere)
• Layer 3 has significant strength for 80 Myr old lithosophere. In some models this
layer yields too.
• Continental Ribbon material replaces layer 1 and layer 2.
Lithosphere mapped into a layered model (Continental vs. Oceanic)